Fix crash in DowncastError Display impl, remove String member

[sbillig]

In the Display impl for DowncastError, fmt() calls self.to_string(), which calls fmt(), which calls self.to_string(), which... causing the following to crash:

    let s = cx.string("hello");
    if let Err(e) = s.downcast::<JsNumber>() {
        eprintln!("downcast error: {}", e);
    }

This commit also removes the String member in DowncastError, eliminating the allocation overhead on creation (which is then paid when/if the error is thrown), and removes the Error::description() impl, which has been deprecated since rust 1.42.

[kjvalencik]

@sbillig This is great! Thanks for fixing this. Would you mind adding a test to test/dynamic and test/napi to prevent a regression in the future?

[sbillig]

@kjvalencik I added a simple test.

[kjvalencik]

@sbillig Thanks for your contribution!